There has long been known a magnetic separation method of placing ferromagnet thin lines (matrix) under a uniform magnetic field and generating a high magnetization gradient near the ferromagnet thin lines. The ferromagnet thin lines can be utilized as a magnetic filter, making it possible to magnetically separate a magnetically attractable substance, which is magnetically attracted to the ferromagnet thin lines, and a non-magnetically attractable substance, which is not magnetically attracted to the ferromagnet thin lines, in a sorting target fluid introduced into a flow path where the ferromagnet thin lines are disposed.
The magnetic separator based on the principle of the magnetic separation method has developed as a high gradient magnetic separator (HGMS; High Gradient Magnetic Separator) and even today, the high gradient magnetic separator has newly been being developed. Specifically, the high gradient magnetic separator has a main object to increase separation accuracy and separation efficiency between the magnetically attractable substance and the non-magnetically attractable substance. For example, one proposed method is blowing pressurized gas in a magnetically attractable substance recovering step in order for particles adsorbed onto the magnetic filter to be detached more easily (see Japanese Patent Application Laid-Open (JP-A) No. 05-123510).
However, the sorting devices utilizing the high gradient magnetic separator that have been proposed so far do not achieve a fundamental improvement in separation accuracy and separation efficiency. Referring to FIGS. 1A to 1C, this situation will be described taking as an example a Jones-type wet high gradient sorting device which is typical as the sorting device. FIGS. 1A to 1C are depictive diagrams depicting a conventional sorting device using a Jones-type wet high gradient magnetic separator.
As illustrated in FIG. 1A, a sorting device 100 includes as main members, a high gradient magnetic separating section 50 including an electromagnet 50a, a magnetic filter 50b, and a magnetic separation flow path 50c, a sorting target fluid introducing flow path 101b coupled to one end of the magnetic separation flow path 50c via an on-off valve 101a and capable of introducing a sorting target fluid into the magnetic separation flow path 50c, a non-magnetically attractable substance discharging flow path 103b coupled to the other end of the magnetic separation flow path 50c via an on-off valve 103a and capable of discharging from the magnetic separation flow path 50c, the sorting target fluid from which any magnetically attractable substance has been magnetically attracted to the magnetic filter 50b, a carrier fluid introducing flow path 104b coupled to the other end of the magnetic separation flow path 50c via an on-off valve 104a and capable of introducing into the magnetic separation flow path 50c, a carrier fluid (e.g., water) capable of carrying the magnetically attractable substance detached from the magnetic filter 50b, and a magnetically attractable substance discharging flow path 105b coupled to the one end of the magnetic separation flow path 50c via an on-off valve 105a and capable of discharging from the magnetic separation flow path 50c, the carrier fluid carrying the magnetically attractable substance detached from the magnetic filter 50b. 
The sorting device 100 is configured to sort the magnetically attractable substance and the non-magnetically attractable substance by separating the magnetically attractable substance and the non-magnetically attractable substance from the sorting target fluid.
First, as illustrated by arrows in FIG. 1A, only the on-off valve 101a of the on-off valves on the one end of the magnetic separation flow path 50c is opened to the magnetic separation flow path 50c in a state that the electromagnet 50a is excited, to introduce into the magnetic separation flow path 50c, the sorting target fluid introduced into the sorting target fluid introducing flow path 101b by means of a pump 101d from a storing section 101c storing the sorting target fluid and have the magnetically attractable substance magnetically attracted to the magnetic filter 50b, and only the on-off valve 103a of the on-off valves on the other end of the magnetic separation flow path 50c is opened to discharge into the non-magnetically attractable substance discharging flow path 103b, the sorting target fluid from which the magnetically attractable substance has been magnetically attracted and recover the sorting target fluid into a non-magnetically attractable substance recovering section 103c (a non-magnetically attractable substance sorting step).
Next, as illustrated by arrows in FIG. 1C, only the on-off valve 104a of the on-off valves on the other end of the magnetic separation flow path 50c is opened to the magnetic separation flow path 50c in a state that the electromagnet 50a is released from excitation, to introduce the carrier fluid into the magnetic separation flow path 50c from the carrier fluid introducing flow path 104b, and only the on-off valve 105a of the on-off valves on the one end of the magnetic separation flow path 50c is opened to have the carrier fluid carry the magnetically attractable substance detached from the magnetic filter 50c to discharge the magnetically attractable substance from the magnetic separation flow path 50c into the magnetically attractable substance discharging flow path 105b and recover the magnetically attractable substance into a magnetically attractable substance recovering section 105c (a magnetically attractable substance sorting step).
In the sorting device 100, by operating each of the on-off valves to repeatedly perform the non-magnetically attractable substance sorting step and the magnetically attractable substance sorting step while switching them based on the magnetic attraction capability of the magnetic filter 50, it is possible to sort the magnetically attractable substance and the non-magnetically attractable substance by separating the magnetically attractable substance and the non-magnetically attractable substance from the sorting target fluid.
In the sorting device 100, however, upon switching between the non-magnetically attractable substance sorting step and the magnetically attractable substance sorting step, the sorting target fluid from which the magnetically attractable substance and the non-magnetically attractable substance have not yet been separated is retained in the magnetic separation flow path 50c, raising a problem with reduction in separation accuracy. Therefore, in order to perform separation with high accuracy, it is necessary to circulate substances that have once undergone sorting, as the sorting target fluid to repeat a similar sorting operation, resulting in reduction in separation efficiency.
Specifically, as illustrated in FIG. 1B, at the time of operation of the non-magnetically attractable substance sorting step, the non-sorted sorting target fluid not having undergone sorting by the magnetic filter 50b is retained on the one end of the magnetic separation flow path 50c. Also, the sorting target fluid from which the magnetically attractable substance has been attracted and which is to be discharged into the non-magnetically attractable substance discharging flow path 103b is retained on the other end of the magnetic separation flow path 50c. In this state, when the non-magnetically attractable substance sorting step is switched to the magnetically attractable substance sorting step by operating each of the on-off valves and the electromagnet 50a, the non-sorted sorting target fluid retained in the magnetic separation flow path 50c, and the sorting target fluid from which the magnetically attractable substance has been attracted and which is to be discharged into the non-magnetically attractable substance discharging flow path 103b are discharged to the magnetically attractable substance discharging flow path 105b. As a result, there is a problem with unintentional inclusion of the non-magnetically attractable substance in the magnetically attractable substance recovering section 105c. 